A mix of self-compacting concrete with remarkable fluidity and without segregation shows a strong accumulation of bubbles on the surface. This accumulation creates a layer of highly porous concrete and not resistant.
The concrete under this surface doesn't show high porosity.
Has anyone had the same problem? Could this be due to excessive kneading times?
Porosity of concrete appears when the amount of water contained in it is too high. The mix should have an almost dry appearance for being as less porous as possible after hardening. If your cement has a high fluidity, was it because of a high amount of water ?
Alain
Hello Carlos,
You mentioned that you did not observe segregation and the mix was fluid. Can you mention the conditions in which this concrete was cast ?
Also the surface layer looks very porous indicating signs of laitance, bleeding. Did you observe either of these when the concrete was cast ?
You can look at the water - binder ratio and the type of superplasticizer you adopted in developing the mix. It can very well be because of an admixture overdose or addition of excess water.
Do let me know if you find out the exact cause of this.
I agree with Dr. Cheruku. It may be because of overdose of the superplasticiser. You should check the specification of the chemical admixture provided by the manufacturer. Moreover, there may be some problems in the mixing sequence and/or procedure you had adopted.
Thank you very much for your answers.
Indeed, it appears that it is due to an excess of the combination of additive and water. Without additive there is no segregation but it isn't enought fluid. Adding additive we have achieved the desired fluidity but we have not reduced the amount of water. We have avoid this problemn by adding more additive while reducing the amount of water while maintaining the right consistency for self-compacting concrete.
Thanks again.
As you know the additives that increase the fluidity, often surround the cement particles and also air inside, thus movement of air bubbles like shuts,cause the more fluidity of concrete. May excess amount of this bubbles moves to surface?
This is typical of an admixture overdose , however the type of admixture must be considered as well , as some superplastisisers incorporates " air de-trainers , specifically designed to remove maximum amount entrapped air from the concrete.
The solution may be to reduce the dosage of the admixture or try an alternative one.
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